Int. J. Corros. Scale Inhib., 2014, 3, no. 3, 177–189
Impact of iron oxide (rust) on the performance of calcium
phosphate scale inhibitors for industrial water systems
Z. Amjad
Division of Mathematics and Sciences, 2020 E. Maple Street, North Canton,
OH 44270, USA
E-mail: [email protected]
Abstract
The inhibition of calcium phosphate precipitation by homo- and copolymers containing
different functional groups has been examined in aqueous solution. It has been found that
polymer performance as calcium phosphate inhibitor depends upon polymer architecture.
The influence of iron oxide (Fe2O3, rust) on the performance of calcium phosphate
inhibitors has also been investigated. Results show that addition of small amount (10 mg to
100 mg) of Fe2O3 to the calcium phosphate supersaturated solution exhibits marked
antagonistic effect on inhibitor performance. Various mechanism(s) have been proposed to
explain the negative impact of Fe2O3 on inhibitor performance.
Keywords: calcium phosphate, precipitation, inhibition, polymers, iron oxide (rust).
Received: May 24, 2014.
doi: 10.17675/2305-6894-2014-3-3-177-189
Introduction
The importance of calcium phosphates as the major constituents of industrial and
pathological mineral deposits has stimulated extensive research on the precipitation and
dissolution of these compounds [1]. In cooling, boiler, and desalination applications, the
insulating nature of scales on equipment surfaces results in decreased system efficiency
and premature equipment failure. In addition to scaling problems, cooling water systems
and boilers constructed of carbon steel also experience corrosion problems caused by
dissolved oxygen [2]. Effective water treatment formulations must control scale, corrosion,
particulate matter, and microbiological growth.
In biological system, examples of calcium containing minerals include dental
calculus, calcified plaques of arteriosclerosis, kidney stones, etc. [3]. Calcium phosphate
deposits have also been reported during pasteurization of milk and sugar refining processes
[4, 5]. In addition, calcium phosphates are widely produced in industry, in such forms as
ceramics, nutrient supplements, medicines, dentrifices, and stabilizers for plastic. They are
used in solid state chemistry as fluorescent lamp phosphors and play a role in waste water
treatment processes.
During the last two decades, the problem of calcium phosphate scaling in industrial
water systems has become increasingly important. Higher orthophosphate levels are being
encountered in cooling water due to increased water reuse, use of low quality make-up
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178
water, and the use of organic phosphonate as scale and corrosion inhibitors which are
degraded to orthophosphate. The increased orthophosphate levels, combined with alkaline
operating conditions can lead to the formation of insoluble calcium phosphate scale
deposits which are normally attributed to hydroxyapatite, Ca5(PO4)3OH, HAP. In cooling
water systems however, it is not the HAP which is initially formed, but instead a precursor
phase is formed which is widely known as amorphous calcium phosphate, Ca 3(PO4)2 [6].
Factors influencing the precipitation of calcium phosphate in aqueous system include, pH,
temperature, ionic strength, process contaminants [7].
Among the methods used to prevent calcium phosphate precipitation are acidification
of water and the use of chemical additives (polymeric and non-polymeric). These methods
prevent and/or retard markedly the precipitation and deposition of calcium phosphate scale
on the heat exchanger surfaces. The acidification method has the disadvantage of exposing
metal surfaces to a corrosive environment while the use of chemical additives seem
promising since only very low concentrations are needed to extend the time required for
the initiation of scale formation. Also, if the acid addition shuts down, an increase in pH
can cause immediate calcium phosphate precipitation. Due to more stringent regulations
and from worker safety perspective addition of chemical additives seems to be the only
reliable solution to the problem.
Inhibition of calcium phosphate precipitation by polymeric additives has been the
subject of intensive research. Chang et al. [8], in their investigation on the evaluation of
double-hydrophilic block copolymers reported that these polymers exhibit excellent
performance as calcium phosphate inhibitors. Amjad [9] investigated the influence of
various homopolymers on the precipitation of calcium phosphates. Results of this study
show that at low supersaturation, addition of low concentrations (few parts per million,
ppm) of low molecular weight (MW) poly(acrylic acid), PAA, significantly retarded the
precipitation of calcium phosphate in aqueous solution. Ohtaka et al. [10] and Amjad et al.
[11] in their investigations of the evaluation of homo- and copolymers concluded that both
the functional group and the ionic charge of the polymer play important roles in inhibiting
precipitation of calcium phosphate.
The influence of suspended matter on the efficacy of polymers as calcium phosphate
inhibitors has been reported. Results of this study reveal that addition of clay particles to
calcium phosphate supersaturated solutions adversely impact the inhibitory property of
polymers because of the adsorption of the polymers on clay particles thereby reducing the
available polymer for scale inhibition [12]. Amjad et al. [13] in another study reported that
addition of clay to silica supersaturated solution did not exhibit any significant effect on
the performance of silica polymerization inhibitors. Recent research in our laboratories has
shown that the presence of small amounts (10 –100 mg) of iron oxide exhibits negative
effect on the performance of calcium sulfate dihydrate and calcium carbonate scale
inhibitors [14, 15]. In the present study we try to discover and explore the influence of iron
oxide, a commonly encountered corrosion product in industrial water system, on the
effectiveness of calcium phosphate scale inhibitors. It is hoped that the data presented in
Int. J. Corros. Scale Inhib., 2014, 3, no. 3, 177–189
179
this paper will enable the water technologists to recognize the impact of suspended matter
in selecting the appropriate additives for their formulations.
Experimental
Materials
Reagents grade chemicals and grade A glassware were used. Stock solution of calcium
chloride dihydrate and disodium hydrogen phosphate were prepared using distilled water,
filtered through 0.22 µm filter paper, and analyzed as described previously [7]. The
additives tested were commercial materials and stock solutions of these additives were
prepared on a dry weight basis. The desired concentrations of the additives were obtained
by dilution. Table 1 lists the compositions, functional group, ionic charge, and acronyms of
additives tested. As noted in Table 1, the additives tested as calcium phosphate inhibitors
vary significantly both in terms of composition and ionic charge. The iron oxide (Fe 2O3)
used in this study was obtained from Fisher Scientific Company. It was characterized as
hematite by X-ray diffraction (JCPDS Phase 33-664, predominantly hematite, with some
minor reflections, which do not correspond to any indexed iron oxides). The particle size
distribution data as measured by Beckman Coulter Counter (Model LS230) revealed that
the majority of Fe2O3 particles fall between 20 and 100 µm with the largest volume% at
~50 µm.
Table 1. Polymers tested.
Polymer
Functional
Group
Ionic Charge*
Molecular
Weight
Acronym
Poly(acrylic acid)
–COOH
Negative
2k
PAA
Poly(maleic acid)
–COOH
Negative
< 1k
PMA
Poly(methacrylic acid)
–COOH
Negative
6k
PMAA
Poly(acrylic acid:
hydroxypropyl acrylate)
–COOH
–COOR
Negative
Neutral
10k
PHP
Poly(acrylic acid:2acrylamido-2-methylpropane
sulfonic acid)
–COOH
–SO3H
Negative
< 15k
PSA
Poly(acrylic acid:2acrylamido-2-methylpropane
sulfonic acid:sulfonated
styrene)
–COOH
–SO3H
Negative
< 15k
PSS
Poly(maleic acid:sulfonated
styrene)
–COOH
–SO3H
Negative
< 10k
PMS
*at the pH of working solution.
Int. J. Corros. Scale Inhib., 2014, 3, no. 3, 177–189
180
Calcium phosphate precipitation protocol
The calcium phosphate precipitation experiments were conducted by adding known volume
of stock solutions of disodium hydrogen phosphate to known volume of water containing
HCO3 / CO32 buffer in a 100 mL glass bottle. To this solution was added with continuous
stirring a known volume of stock solution of calcium chloride. In experiments involving
the additive, a known volume of stock solution of inhibitor was added to the phosphate
solution prior to the addition of calcium chloride solution. The pH of the calcium
phosphate solutions were adjusted to 8.35 ± 0.05 with dilute sodium hydroxide and
experimental solutions were stored in a water bath maintained at 50°C. At known times,
the solution was filtered through 0.45 µm filter paper and phosphate concentration was
determined by standard spectrophotometric method. Experiments involving the influence
of iron oxide were done by adding a known amount of iron oxide to calcium phosphate
supersaturated solution and manually mixing the solution for 60 seconds. The experimental
conditions employed in the present study include: 110 mg/L Ca, 13.0 mg/L phosphate, pH
8.35, 50°C. The results reported here had good reproducibility (± 6%).
The polymer efficacy as a calcium phosphate inhibitor was calculated using the
following equation:
Percent Inhibition (%I ) = [(PO4)e – (PO4)f] / [(PO4)i – (PO4)f],
(1)
where:
(PO4)e = concentration of phosphate in the filtrate in the presence of inhibitor at known
time;
(PO4)i = concentration of phosphate at the beginning of experiment; and
(PO4)f = concentration of phosphate in the filtrate in the absence of inhibitor at 20 hr.
Results and Discussion
During the past two decades, a number of natural or biodegradable and synthetic additives
has been developed and is currently being used as component of water treatment
formulations. The role of these additives (polymeric and non-polymeric in such
formulations is twofold: a) to inhibit the precipitation of scale forming salts such as
CaCO3, CaF2, BaSO4, Mg(OH)2, etc., and b) to disperse the suspended matter such as clay,
iron oxide, organic debris, etc. In the former case, the additive inhibits the precipitation of
scale forming salt, by adsorption onto the scale crystallites and prevents them from further
growth. The additives that fall into this category are usually low molecular weight (MW)
homopolymers containing carboxyl group (–COOH). Dispersants on the other hand, are
copolymers containing different functional groups i.e., –COOH, sulfonic acid (–SO3H);
ester (–COOR); s-acrylamide (–CONHR). These polymers function by adsorbing onto
particles such as iron oxide, silt, clay, etc., present in recirculating waters and preventing
them from settling on the equipment surfaces. In the present investigation we have tested
polymers from both categories as calcium phosphate inhibitors.
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181
Performance of polymers
Polymer dosage: By conducting a series of precipitation experiments it is possible to
describe the dosage – performance profile relationship for a given polymer as illustrated in
Figure 1. As shown the addition of relatively small dosage (i.e., 5.0 ppm) of poly(acrylic
acid), PAA, exhibits negligible effect on calcium phosphate precipitation reaction.
However, as the PAA dosage is increased from 5.0 to 10.0 ppm, the performance of PAA
is substantially improved. For example, solution phosphate concentration in the presence
of 10 ppm of PAA at 20 hr is 6.60 compared to 1.95 and 0.45 mg/L obtained in the
presence of 5.0 and 0 ppm of PAA, respectively. As shown in Figure 1 increasing the PAA
dosage by 50% (i.e., from 10 to 15 ppm) results in ~80% increase in solution phosphate
concentration, and at 30 ppm, >90% inhibition is achieved. Figure 2 presents plots of
percent calcium phosphate inhibition (%I ) at 20 hr as calculated according to Equation 1,
as a function of PAA dosage. It is apparent that calcium phosphate inhibition increases
with increasing PAA dosage. It is worth noting that similar polymer dosage – performance
profile as noted in Figure 2 has been previously reported for CaCO3, CaF2, and BaSO4
scaling systems [16, 17, 18].
14
Phosphate, mg/L
12
10
0 ppm
8
2.5 ppm
5.0 ppm
6
10 ppm
4
15 ppm
30 ppm
2
0
0
5
10
15
20
Time, hr
Figure 1. Calcium phosphate precipitation as a function of time and in the presence of varying
concentration of poly(acrylic acid), PAA.
Polymer composition: The effect of polymer composition on calcium phosphate
precipitation was investigated by a series of precipitation experiments conducted under
similar experimental conditions in the presence of copolymers of varying compositions.
Excellent examples of substitution of –COOH group with other monomers containing
different functional groups are presented in Figure 3, in which the %I is plotted for acrylic
acid:2-acrylamido-2-methyl propane sulfonic acid, PSA; acrylic acid:2-acryalamido-2methylpropane sulfonic acid:sulfonated styrene, PSS; acrylic acid:hydroxylpropyl acrylate,
Int. J. Corros. Scale Inhib., 2014, 3, no. 3, 177–189
182
PHA; and maleic acid:sulfonated styrene, PMS. It can be seen that polymers containing 2
or 3 monomers with different functional groups perform better than homopolymers
containing –COOH i.e., poly(acrylic acid), PAA; poly(maleic acid), PMA;
poly(methacrylic acid), PMAA. It is interesting to point out that whereas copolymers
compared to homopolymers show excellent performance as calcium phosphate inhibitors,
these homopolymers on the other hand, have been shown to perform better than
copolymers for other calcium containing minerals such as CaCO3, CaF2, and BaSO4 [16,
17, 18].
100
% Inhibition
80
60
40
20
0
2.5
5.0
10.0
15.0
30.0
PAA, ppm
Figure 2. Plot of calcium phosphate inhibition (%I ) as a function of varying concentration of
PAA.
100
% Inhibition
80
60
40
20
0
PAA PMA PMAA PSA
PSS
PHA PMS
Figure 3. Plots of % inhibition in the presence of 10 ppm of various homo- and copolymers at
20 hr.
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183
Effect of filter size on polymer performance: It is generally agreed that both,
dispersion (or de-agglomeration) of suspended particles and scale inhibition processes are
believed to play an important role in the success of the phosphate-based treatment program
for cooling water system. Although many factors play a role in determining the rate of
scale formation, one key aspect is maintaining the size of the particulates below a certain
minimum [19]. According to Stoke’s law, the settling rate of spherical particles is
proportional to the square of its diameter in quite solution. Particles with less 0.1 µm
diameter will settle at about 1/20th the rate of 0.45 µm particles, if no secondary effects are
operative. In addition, rapid growth of particles can occur after a certain critical diameter is
reached [20]. Polymeric inhibitors serve to limit particle size, thereby controlling
additional growth. To study the impact of polymeric scale inhibitors on the growth of
calcium phosphate particles, a series of experiments were carried out in which different
filter paper size was used to filter the solutions. As indicated in Table 2, PSS performs the
best at minimizing particle growth.
Table 2. Particle size effects on the performance of calcium phosphate inhibitors.
Polymer
% Inhibition
0.10 µm
% Inhibition
0.22 µm
% Inhibition
0.45 µm
PAA
PHPSA
PSS
19
32
61
28
55
80
44
72
97
Influence of iron oxide on polymers performance
Feed water used in many industrial processes comes from various sources such as rivers,
ponds, lakes, and contains a variety of soluble and insoluble impurities (or suspended
particles). Typical suspended particles include, metal oxides/hydroxides, clay, silt, metal
carbonates, algae, bacteria, and viruses. It is generally known that higher concentrations of
suspended particles not only lead to high turbidity but also provide high surface area for
adsorption of dissolved species including tannins, fulvic acid, humic acid, and additives
used to control scale formations.
Effect of Mixing Time: In order to understand the influence of mixing time after the
addition of Fe2O3 to the calcium phosphate supersaturated solutions, several precipitation
experiments were carried out in the presence of 10 ppm of PSS, and 10 mg of Fe2O3.
Results presented in Figure 4 show that mixing time exhibits marked negative effect on the
performance of PSS. For example, %I values obtained at 20 hr, after no mixing (0 min),
0.50 min and 1.0 min are 76%, 58%, and 34% respectively, compared to 98% obtained at
20 hr in the absence of iron oxide. It is interesting to note that increasing the mixing time
by three fold i.e., from 1 in to 3 min does not show any significant impact (i.e., 34% to
30%) on the performance of PSS, suggesting that mixing time plays an important role in
the adsorption of PSS on iron oxide particles and/or removal of PSS from the calcium
phosphate supersaturated solution.
Int. J. Corros. Scale Inhib., 2014, 3, no. 3, 177–189
184
100
% Inhibition
80
60
40
20
0
none
0.00
0.50
Time, min
1.00
3.00
Figure 4. Effect of mixing time on the performance of 10 ppm copolymer (PSS) in the
presence of 100 mg iron oxide.
Effect of iron oxide concentration on polymers performance: To understand the
impact of iron oxide concentration on the performance of Ea polymer, a series of
precipitation experiments was carried out in the presence of 10 ppm of PSS and varying
D a function of time for
amounts of Fe2O3. Figure 5 presents phosphate concentration as
experiments carried out in the absence of PSS and in the presence of varying amounts of
Fe2O3. It can be seen that in the presence of 0 ppm PSS, the addition of 25 or 100 mg
Fe2O3 to the calcium phosphate supersaturated solutions does not exhibit any significant
effect on the rate of calcium phosphate precipitation in the absence of PSS (curves A, B,
C). Figure 5 also presents phosphate concentration as a function of time collected for
experiments conducted in the presence of 10 ppm PSS and 0 mg and 100 mg of Fe2O3. It is
apparent from the data that in the absence of Fe2O3, calcium phosphate precipitation is
completely inhibited for at least 20 hr. However, as shown in Figure 5 addition of 100 mg
Fe2O3 to the calcium phosphate supersaturated solution containing 10 ppm PSS exhibits
marked negative impact on the performance of PSS. For example, solution phosphate
concentration obtained at 20 hr in the absence of Fe2O3 and 10 ppm of PSS are 12.4 mg/L
(curve D) compared to 4.85 mg/L (curve E) obtained in the presence of Fe2O3, a decrease
of ~ 60% in phosphate concentration. The observed decrease (from 12.8 to 4.85 mg/L) in
phosphate concentration in the presence of Fe2O3 may be attributed to the depletion of PSS
via adsorption on Fe2O3 particles, thus resulting in poor performance of PSS.
To overcome the negative impact of Fe2O3 on the performance of PSS, a series of
precipitation experiments were conducted at constant supersaturation and in the presence
of 100 mg Fe2O3 and varying dosages of PSS. Results of these experiments are presented in
Figure 6. It is evident that in the absence and presence of 100 mg Fe2O3, calcium
phosphate inhibition increases with increasing PSS concentrations. It is also worth noting
that to achieve similar %I value (i.e., >90%) it would require ~9.0 and ~25 ppm PSS in the
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185
absence and presence of Fe2O3, respectively. Thus, to overcome the antagonistic influence
of Fe2O3 on the performance of inhibitor, additional amount, almost by ~ 2.8x of PSS is
required to inhibit calcium phosphate inhibition. It is also worth mentioning that in calcium
sulfate dihydrate scaling system, to achieve >85% inhibition, it required ~2.5x the amount
of inhibitor i.e., poly(acrylic acid) in the presence of Fe2O3 [14].
14
Phosphate, mg/L
12
10
A
8
B
6
C
D
4
E
2
0
0
5
10
15
20
25
Time, hr
Figure 5. Plots of phosphate concentrations as a function of time in the presence and absence
of PSS and iron oxide (curve A, no polymer, no iron oxide; curve B, no polymer, 25 mg iron
oxide; curve C, no polymer, 100 mg iron oxide; curve D, 10 ppm of polymer (PSS), no iron
oxide; curve E, 10 ppm of PSS, 100 mg iron oxide).
100
% Inhibition
80
60
0 mg Fe2O3
100 mg Fe2O3
40
20
0
0
5
10
15
20
25
PSS, ppm
Figure 6. Plots of % inhibition at 20 hr in the presence of 10 ppm PSS and 0 and 100 mg iron
oxide.
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186
It is known that whereas homopolymers containing the –COOH group are effective
inhibitors for various scales i.e., CaF2, CaCO3, and BaSO4 [16, 17, 18], copolymers
containing –COOH and other functional groups such as –COOR, –SO3H, –CONR1R2, etc.,
are excellent inhibitors for calcium phosphate and are also effective dispersants for iron
oxide [7, 21]. In order to investigate the influence of Fe2O3 on the performance of homo-,
co-, and terpolymers, several experiments were carried out in the presence of 10 ppm of
polymers and 100 mg of Fe2O3. The polymers tested include: PAA, PMA, PSA, PSS,
PMS, and PHP. Results obtained in the presence of 10 ppm of polymers and 100 mg
Fe2O3, are presented in Figure 7. It can be seen that under similar experimental conditions
all polymers lose their inhibition efficacy by ~80%. The decreased %I values obtained in
the presence of Fe2O3 reflect the depletion of polymer concentration in calcium phosphate
supersaturated solutions via adsorption onto Fe2O3 particles.
100
0 mg Fe2O3
100 mg Fe2O3
% Inhibition
80
60
40
20
0
PAA PMA PMAA PSA
PSS
PHA
PMS
Figure 7. Effect of iron oxide on the performance of various homo- and copolymers at 20 hr,
10 ppm polymer.
Proposed mechanism(s) for performance loss of polymers by Fe2O3
The marked antagonistic effect by Fe2O3 on the performance of various polymeric
additives may be attributed to one or more of the following mechanisms:
a) Interactions of Fe(III)-hydroxo complexes with additives: It is likely that Fe(III)
hydroxo complexes (i.e., Fe(OH)2+, Fe(OH)2+, Fe2(OH)24+, etc., formed in the aqueous
solution interact with anionic additive molecules, thus resulting in decreased solution
concentration of the additive.
b) Effect of Fe(III) ions: It has been reported [22, 23] that the presence of low
concentrations (~ 1 to 3 mg/L) of Fe(III) ions exhibits inhibitory effect on calcium
phosphate precipitation from supersaturated solutions. In precipitation experiments
involving Fe2O3, the Fe(III) concentration in the filtrate at 20 hr was found to be
~ 0.02 mg/L. To check if Fe(III) shows any effect (synergistic or antagonist) on the
Int. J. Corros. Scale Inhib., 2014, 3, no. 3, 177–189
187
performance of additive, calcium phosphate precipitation experiments were carried out
in the presence of 10 ppm of PSS and 0.02 and 0.1 mg/L of Fe(III). Results of these
experiments show that Fe(III) under the conditions employed did not exhibit any
significant influence on the performance of PSS.
c) Interactions of Fe(III) with anionic inhibitors: The formation of insoluble salts of
–COOH containing polymers with metal ions such as Ca, Mg, Ba, etc., has been
reported [24]. It is possible that trace levels of Fe(III) generated in experiments
involving Fe2O3 in CaCO3 supersaturated solutions may form insoluble salt with anionic
polymers, thus reducing polymer concentration in solution.
d) Removal of Additives via Adsorption on Fe2O3: The influence of homo-, co-, and
terpolymers as dispersants for suspended matter (i.e., iron oxide, clay, calcium
phosphate, etc.) has been investigated [21]. Results of these studies reveal acrylic acid
and maleic acid based homo- and copolymers disperse suspended matter to a varying
degree. In general, co- and terpolymers are better dispersants than homopolymers. The
fact that additive performance is affected by the amount of Fe2O3 present in the
supersaturated solution suggest that additive concentration in solution is depleted via
adsorption onto Fe2O3 particles thus effectively decreases the solution concentration of
the additive.
Based on the data presented in the present study, it is likely that one or more of the
proposed mechanism(s) is responsible for the marked decrease in additive performance as
CaCO3 inhibitor. Studies are currently underway to better understand the additive
performance loss and the results of these studies will be presented in future papers.
Summary
The results presented in this paper support the following conclusions on the effect of iron
oxide in inhibiting calcium phosphate by polymers with varying composition and
containing different functional groups:
 Carboxyl group containing homopolymers i.e., poly(acrylic acid), poly(maleic acid),
poly(methacrylic acid) show poor performance as calcium phosphate inhibitors.
 Based on the data presented, the order of polymer performance in terms of decreasing
performance is: copolymers > homopolymers.
 Inhibition of calcium phosphate precipitation increases with increasing polymer
concentration.
 Addition of iron oxide particles to calcium phosphate supersaturated markedly reduces
polymer performance as calcium phosphate inhibitor.
 Data presented in this study further reveal that the negative influence shown by the
addition of Fe2O3 to calcium phosphate supersaturated solution can be overcome by
increasing the inhibitor concentration.
Int. J. Corros. Scale Inhib., 2014, 3, no. 3, 177–189
188
Acknowledgement
The author thanks Walsh University for support to carry out this investigation.
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